// SPDX-License-Identifier: GPL-2.0/* * Copyright (C) 2022-2024 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved. */ #include <linux/array_size.h>#include <linux/minmax.h>#include <vdso/datapage.h>#include <vdso/getrandom.h>#include <vdso/unaligned.h>#include <asm/vdso/getrandom.h>#include <uapi/linux/mman.h>#include <uapi/linux/random.h> #undef PAGE_SIZE#undef PAGE_MASK#define PAGE_SIZE (1UL << CONFIG_PAGE_SHIFT)#define PAGE_MASK (~(PAGE_SIZE - 1)) #define MEMCPY_AND_ZERO_SRC(type, dst, src, len) do {				\	while (len >= sizeof(type)) {						\		__put_unaligned_t(type, __get_unaligned_t(type, src), dst);	\		__put_unaligned_t(type, 0, src);				\		dst += sizeof(type);						\		src += sizeof(type);						\		len -= sizeof(type);						\	}									\} while (0) static void memcpy_and_zero_src(void *dst, void *src, size_t len){	if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {		if (IS_ENABLED(CONFIG_64BIT))			MEMCPY_AND_ZERO_SRC(u64, dst, src, len);		MEMCPY_AND_ZERO_SRC(u32, dst, src, len);		MEMCPY_AND_ZERO_SRC(u16, dst, src, len);	}	MEMCPY_AND_ZERO_SRC(u8, dst, src, len);} /** * __cvdso_getrandom_data - Generic vDSO implementation of getrandom() syscall. * @rng_info:		Describes state of kernel RNG, memory shared with kernel. * @buffer:		Destination buffer to fill with random bytes. * @len:		Size of @buffer in bytes. * @flags:		Zero or more GRND_* flags. * @opaque_state:	Pointer to an opaque state area. * @opaque_len:		Length of opaque state area. * * This implements a "fast key erasure" RNG using ChaCha20, in the same way that the kernel's * getrandom() syscall does. It periodically reseeds its key from the kernel's RNG, at the same * schedule that the kernel's RNG is reseeded. If the kernel's RNG is not ready, then this always * calls into the syscall. * * If @buffer, @len, and @flags are 0, and @opaque_len is ~0UL, then @opaque_state is populated * with a struct vgetrandom_opaque_params and the function returns 0; if it does not return 0, * this function should not be used. * * @opaque_state *must* be allocated by calling mmap(2) using the mmap_prot and mmap_flags fields * from the struct vgetrandom_opaque_params, and states must not straddle pages. Unless external * locking is used, one state must be allocated per thread, as it is not safe to call this function * concurrently with the same @opaque_state. However, it is safe to call this using the same * @opaque_state that is shared between main code and signal handling code, within the same thread. * * Returns:	The number of random bytes written to @buffer, or a negative value indicating an error. */static __always_inline ssize_t__cvdso_getrandom_data(const struct vdso_rng_data *rng_info, void *buffer, size_t len,		       unsigned int flags, void *opaque_state, size_t opaque_len){	ssize_t ret = min_t(size_t, INT_MAX & PAGE_MASK /* = MAX_RW_COUNT */, len);	struct vgetrandom_state *state = opaque_state;	size_t batch_len, nblocks, orig_len = len;	bool in_use, have_retried = false;	void *orig_buffer = buffer;	u64 current_generation;	u32 counter[2] = { 0 }; 	if (unlikely(opaque_len == ~0UL && !buffer && !len && !flags)) {		struct vgetrandom_opaque_params *params = opaque_state;		params->size_of_opaque_state = sizeof(*state);		params->mmap_prot = PROT_READ | PROT_WRITE;		params->mmap_flags = MAP_DROPPABLE | MAP_ANONYMOUS;		for (size_t i = 0; i < ARRAY_SIZE(params->reserved); ++i)			params->reserved[i] = 0;		return 0;	} 	/* The state must not straddle a page, since pages can be zeroed at any time. */	if (unlikely(((unsigned long)opaque_state & ~PAGE_MASK) + sizeof(*state) > PAGE_SIZE))		return -EFAULT; 	/* Handle unexpected flags by falling back to the kernel. */	if (unlikely(flags & ~(GRND_NONBLOCK | GRND_RANDOM | GRND_INSECURE)))		goto fallback_syscall; 	/* If the caller passes the wrong size, which might happen due to CRIU, fallback. */	if (unlikely(opaque_len != sizeof(*state)))		goto fallback_syscall; 	/*	 * If the kernel's RNG is not yet ready, then it's not possible to provide random bytes from	 * userspace, because A) the various @flags require this to block, or not, depending on	 * various factors unavailable to userspace, and B) the kernel's behavior before the RNG is	 * ready is to reseed from the entropy pool at every invocation.	 */	if (unlikely(!READ_ONCE(rng_info->is_ready)))		goto fallback_syscall; 	/*	 * This condition is checked after @rng_info->is_ready, because before the kernel's RNG is	 * initialized, the @flags parameter may require this to block or return an error, even when	 * len is zero.	 */	if (unlikely(!len))		return 0; 	/*	 * @state->in_use is basic reentrancy protection against this running in a signal handler	 * with the same @opaque_state, but obviously not atomic wrt multiple CPUs or more than one	 * level of reentrancy. If a signal interrupts this after reading @state->in_use, but before	 * writing @state->in_use, there is still no race, because the signal handler will run to	 * its completion before returning execution.	 */	in_use = READ_ONCE(state->in_use);	if (unlikely(in_use))		/* The syscall simply fills the buffer and does not touch @state, so fallback. */		goto fallback_syscall;	WRITE_ONCE(state->in_use, true); retry_generation:	/*	 * @rng_info->generation must always be read here, as it serializes @state->key with the	 * kernel's RNG reseeding schedule.	 */	current_generation = READ_ONCE(rng_info->generation); 	/*	 * If @state->generation doesn't match the kernel RNG's generation, then it means the	 * kernel's RNG has reseeded, and so @state->key is reseeded as well.	 */	if (unlikely(state->generation != current_generation)) {		/*		 * Write the generation before filling the key, in case of fork. If there is a fork		 * just after this line, the parent and child will get different random bytes from		 * the syscall, which is good. However, were this line to occur after the getrandom		 * syscall, then both child and parent could have the same bytes and the same		 * generation counter, so the fork would not be detected. Therefore, write		 * @state->generation before the call to the getrandom syscall.		 */		WRITE_ONCE(state->generation, current_generation); 		/*		 * Prevent the syscall from being reordered wrt current_generation. Pairs with the		 * smp_store_release(&_vdso_rng_data.generation) in random.c.		 */		smp_rmb(); 		/* Reseed @state->key using fresh bytes from the kernel. */		if (getrandom_syscall(state->key, sizeof(state->key), 0) != sizeof(state->key)) {			/*			 * If the syscall failed to refresh the key, then @state->key is now			 * invalid, so invalidate the generation so that it is not used again, and			 * fallback to using the syscall entirely.			 */			WRITE_ONCE(state->generation, 0); 			/*			 * Set @state->in_use to false only after the last write to @state in the			 * line above.			 */			WRITE_ONCE(state->in_use, false); 			goto fallback_syscall;		} 		/*		 * Set @state->pos to beyond the end of the batch, so that the batch is refilled		 * using the new key.		 */		state->pos = sizeof(state->batch);	} 	/* Set len to the total amount of bytes that this function is allowed to read, ret. */	len = ret;more_batch:	/*	 * First use bytes out of @state->batch, which may have been filled by the last call to this	 * function.	 */	batch_len = min_t(size_t, sizeof(state->batch) - state->pos, len);	if (batch_len) {		/* Zeroing at the same time as memcpying helps preserve forward secrecy. */		memcpy_and_zero_src(buffer, state->batch + state->pos, batch_len);		state->pos += batch_len;		buffer += batch_len;		len -= batch_len;	} 	if (!len) {		/* Prevent the loop from being reordered wrt ->generation. */		barrier(); 		/*		 * Since @rng_info->generation will never be 0, re-read @state->generation, rather		 * than using the local current_generation variable, to learn whether a fork		 * occurred or if @state was zeroed due to memory pressure. Primarily, though, this		 * indicates whether the kernel's RNG has reseeded, in which case generate a new key		 * and start over.		 */		if (unlikely(READ_ONCE(state->generation) != READ_ONCE(rng_info->generation))) {			/*			 * Prevent this from looping forever in case of low memory or racing with a			 * user force-reseeding the kernel's RNG using the ioctl.			 */			if (have_retried) {				WRITE_ONCE(state->in_use, false);				goto fallback_syscall;			} 			have_retried = true;			buffer = orig_buffer;			goto retry_generation;		} 		/*		 * Set @state->in_use to false only when there will be no more reads or writes of		 * @state.		 */		WRITE_ONCE(state->in_use, false);		return ret;	} 	/* Generate blocks of RNG output directly into @buffer while there's enough room left. */	nblocks = len / CHACHA_BLOCK_SIZE;	if (nblocks) {		__arch_chacha20_blocks_nostack(buffer, state->key, counter, nblocks);		buffer += nblocks * CHACHA_BLOCK_SIZE;		len -= nblocks * CHACHA_BLOCK_SIZE;	} 	BUILD_BUG_ON(sizeof(state->batch_key) % CHACHA_BLOCK_SIZE != 0); 	/* Refill the batch and overwrite the key, in order to preserve forward secrecy. */	__arch_chacha20_blocks_nostack(state->batch_key, state->key, counter,				       sizeof(state->batch_key) / CHACHA_BLOCK_SIZE); 	/* Since the batch was just refilled, set the position back to 0 to indicate a full batch. */	state->pos = 0;	goto more_batch; fallback_syscall:	return getrandom_syscall(orig_buffer, orig_len, flags);} static __always_inline ssize_t__cvdso_getrandom(void *buffer, size_t len, unsigned int flags, void *opaque_state, size_t opaque_len){	return __cvdso_getrandom_data(__arch_get_vdso_rng_data(), buffer, len, flags, opaque_state, opaque_len);}